Possibly contamination from the back side. This is not usually a problem with 6010, but if there are oils on the back side, it can do this. Another possibility is the flux is contaminated, perhaps accidentally caught overspray from WD-40 or some other substance. It could also just be a junk brand. Or crappy base metal you're welding. Some steel is just hub caps and brake rotors from a 55 Chevy remelted in a Chinese furnace.